Usually you would plug the USB cable into the camera and use Windows Explorer (My Computer) to copy these files to your computer. After you have moved the image files, select the images you want in a preview program such as Windows Print and Fax Viewer. Then open your email program and go to the attach button and select those images you wish to include.
The Cannon SD550 can be purchased directly at Cannon, online retailers such as Amazon and eBay. You might also want to see if your local photographic store stocks or can order the Cannon SD550.
It is compatible with Canon PowerShot SD400, SD430, SD450, SD500, SD550, and SD700IS Digital ELPHs.
The 900mAh battery will work fine - it just gives your phone an extended time between charges. I've replaced the original 700mAh battery in my phone for a 950mAh one - and get about 40% extra talk & standby times.